The present invention relates to a magneto-rheological (xe2x80x9cMRxe2x80x9d) fluid damper, and more particularly, to a linear-acting fluid damper suitable for vibration damping in a vehicle suspension.
Magneto-rheological fluids are materials that respond to an applied magnetic field with a change in rheological behavior (i.e., change in formation and material flow characteristics). The flow characteristics of these MR fluids change several orders of magnitude within milliseconds when subjected to a suitable magnetic field. In particular, magnetic particles noncolloidally suspended in fluid align in chainlike structures parallel to the applied magnetic field, thus increasing the viscous characteristics, or apparent viscosity, of the MR fluid.
Devices, such as controllable dampers and struts, benefit from the controllable viscosity of MR fluid. For example, linearly acting MR fluid dampers are used in vehicle suspension systems as vibration dampers. At low levels of vehicle vibration, the MR fluid damper lightly damps the vibration, providing a more comfortable ride, by applying a low magnetic field or no magnetic field all to the MR fluid. At high levels of vehicle vibration, the amounts of damping can be selectively increased by increasing the density of the magnetic field and by applying control integration into vehicle suspension systems that sense and respond to vehicle load, road surface condition, and driver preference by adjusting a suspension performance accordingly.
Generally, current linearly acting MR fluid dampers are based on a monotube design with a coil positioned in a piston of the damper. In the monotube design, the piston moves within the fixed length cylindrical reservoir in response to force from a piston rod that extends outside of the cylinder. The monotube approach simplifies sealing of the MR fluid within the monotube reservoir; however, monotube dampers may experience reliability problems arising from the electrical wiring leading to the coil, etc., necessary for generating a magnetic field in or around parts of the piston. Typically, the electrical wiring passes up through a passage in the piston rod to a coil in the piston. Elaborate assembly procedures are required to seal this passage. Even if adequately sealed, the electrical wiring may flex with the movement of the piston, sometimes resulting in breakage of the wires.
In some dampers, it is known to reduce failure from wire flexing by holding the coil stationary with respect to a portion of the reservoir of (e.g., either the inner or outer tube). In particular, in U.S. Pat. No. 5,277,281, a reduced diameter piston moves within a reduced diameter inner tube. A coil, separate from the piston, acts as a valve control for a flow passage between the inner and outer tubes, rather than a coil integral to the piston controlling flow past the piston. Although wire flexing is reduced, the reduced piston diameter correspondingly reduces damping. Also, leaks due to introducing wiring into the reservoir are not avoided.
Consequently, a significant need exists for an MR fluid damper that is more reliable and inexpensive to manufacture while being tolerant of side loads on the damping components and furthermore, reduces the likelihood of pressure leaks from the MR fluid reservoir.
The present invention provides an MR fluid damper that is of a simpler construction then known dampers and can be manufactured for less cost. However, the MR fluid damper design of the, present invention provides an improved, more reliable performance and substantially increases the reliability of the electrical connection to the coil. One aspect of the invention provides an improved magneto-rheological (xe2x80x9cMRxe2x80x9d) fluid damper including a damper cylinder containing a volume of MR fluid. The cylinder includes an inner surface. A piston assembly is disposed in the cylinder and has an outer surface slidably contacting with the cylinder inner surface. The piston assembly includes a flow gap formed therein and an external coil surrounding a portion of the cylinder, the external coil capable of generating a magnetic field across at least a portion of the flow gap. A pair of ferromagnetic rings are provided, one of which is positioned above and the other of which is positioned below the external coil for directing the magnetic field or flux through the flow gap.
Other aspects of the invention provide a damper wherein the piston assembly includes a first portion having a first diameter and a second portion having a second diameter, the first diameter being less than the second diameter, the second portion including the outer surface in contact with the cylinder inner surface. The MR damper flow gap can be formed along the first portion of the piston assembly. The second portion of the piston assembly can include a plurality of openings. The MR damper can further include a piston rod, a major portion of which is disposed in the cylinder and wherein the piston assembly is secured to an inner end of the piston rod. The piston assembly can be secured to the rod by a pin. The pin can secure the first portion of the piston assembly to the inner end of the piston rod. The outer surface of the second portion of the piston assembly may include a wear resistant coating. The wear resistant coating can include a nickel plating. The wear resistant coating can include an iron alloy including from about 27-50% cobalt and alternately, about 2% vanadium. The wear resistant coating can be sprayed onto the outer surface of the second portion of the piston assembly. The outer surface may be turned and roller burnished. The MR damper may further include a first pair of retaining members positioned in grooves formed in the piston rod at positions above and below the piston assembly and a Belleville spring positioned between one of the first pair of retaining members and the piston assembly to secure the piston assembly to the piston rod. The retaining members may be retaining rings. The extending end of the piston rod opposite the inner end is secured to a housing of the damper by a threaded member. The extending end of the piston rod opposite the inner end may be secured to a housing of the damper by a second pair of retaining members positioned on the inside and the outside of the housing and a Belleville spring can be positioned between one of the pair of retaining members and the housing to secure the piston rod to the housing. The piston rod can be a solid rod. The cylinder can be made of a material that saturates at about 0.5 to about 2 Tesla. The MR damper can further include a gas cup slidingly contained within the cylinder, the gas cup defining a gas chamber containing a gas in one portion of the cylinder, the gas cup configured to seal the MR fluid from the gas chamber. The ferromagnetic rings may include a pair of inner bearings for allowing the ferromagnetic rings and the coil positioned therebetween to slidingly contact the cylinder. The vertical span of the coil and ring assembly may be a length at least equal to a vertical span of the piston.